1. Field of the Invention
The present invention relates to a transmission apparatus such as a modulator-demodulator (modem).
A modem has been used in the past by connecting it to an analog circuit and using it to perform data transfer. In this case, while instantaneous dropouts of the circuit can occur, depending upon the condition of the circuit, it is necessary to have a means of preventing such instantaneous circuit dropouts from disturbing the data transfer.
2. Description of Related Art
In a modem in the past, before starting data communication, a training signal is sent and received between a modem and the modem on the other end to perform synchronization lock-in processing which establishes synchronization therebetween. By performing synchronization lock-in processing, the modem is set to a condition which is optimized for the condition of the circuit, enabling normal reception of data. After completion of the synchronization lock-in processing, data communication is started. In the above-noted synchronization lock-in processing, automatic equalizer tap coefficients, AGC circuit coefficients, and the like within the modem are set in response to the condition of the circuit over which the signal is transferred.
There are cases in which the data being communicated is interrupted by what is known as an instantaneous dropout. This instantaneous dropout can occur by a physical breaking of the communication line, by manual twisting of a communication line the covering of which has become deteriorated, and by a poor connection caused by externally applied vibration, temperature variation, or the like. A judgment with regard to the occurrence of this instantaneous dropout can be known in the case of a modem by whether or not a carrier signal is being sent from the modem of the other party.
As methods for handling an instantaneous dropout, a first method is that in which synchronization lock-in is started again immediately after the detection of recovery from the instantaneous dropout, and second method being the recognition of the instantaneous dropout, and the holding of the modulation-demodulation section of the modem in the condition that it was in immediate prior to the instantaneous dropout. Specifically, the values of the settings of each part of the modem which were set by synchronization lock-in are saved in a memory. These values include, for example, automatic equalizer tap coefficients and AGC circuit coefficients. When the carrier signal is detected once again and it is judged that recovery has been made from the instantaneous dropout, the setting values which had been stored in the memory are re-set into the various parts of the modem, and data reception operation is restarted under the conditions which were in effect immediately prior to the occurrence of the instantaneous dropout. In this manner, by temporarily saving the setting value of each part and re-setting these values at the point at which recovery from the instantaneous dropout is achieved, it is possible to restore data communication quickly without having to reperform synchronization lock-in after recovery from the instantaneous dropout.
However, in the above-described methods of the past, the following problems exist.
Because telephones lines in the past were generally analog lines and the modems were also designed as analog modems to suit these lines, instantaneous dropouts occurring thereon were mostly within 1 second, so that it was sufficient for a modem to be able to tolerate an instantaneous dropout of this length. However, with the digitization of telephone lines in recent years, the mode of occurrence of instantaneous dropouts has changed, and because of the framing of data to be transferred on digital lines, instantaneous dropouts of a length from 1 to 4 seconds, caused by loss of frame synchronization often occur.
In addition, in a modem for the analog lines of the past, because the modem was designed for a recovery from an instantaneous dropout of approximately 1 second, if an instantaneous dropout occurs over a period of greater than 1 second, synchronization lock-in will be started once again with respect to the other modem, so that in the first method even if the synchronization lock-in is redone from the beginning a training period of approximately several seconds was sufficient. With the appearance of high-speed modems in recent years, however, the training period is greatly lengthened, so that in the case in which the synchronization lock-in processing is performed once again, up to approximately 20 seconds is required from the time of the occurrence of the instantaneous dropout to the re-establishment of communication. However, a problem occurred in that, when approximately 20 seconds have elapsed from the time of the occurrence of an instantaneous dropout, the network times out, resulting in the network itself going down. In this case, because data communication itself is terminated, it was not possible to quickly restart data communication.
The modulation-demodulation sections of modems in recent years have been implemented by LSI devices, with the settings of the functions thereof fixed, making it extremely difficult to add new functions to or change the functions of such LSI-implemented modulation-demodulation sections. For this reason, in the case of a modem of the past which does not accommodate the above-noted instantaneous dropout countermeasures, if an instantaneous dropout occurred the apparatus would be temporarily reset regardless of the length thereof, making it necessary to perform synchronization lock-in once again.
In the case of the above-described second method as well, it was necessary to provide a memory for the saving of a variety of parameters and the like, resulting in an increase in the scale of the hardware and manufacturing cost, in addition to the problem of a limitation in memory resources in the case of the above-noted LSI-implemented modem. In addition, a problem existed in that the sequential control for the purpose of the processing of saving into memory was complex and difficult.